Y: Want to see a picture of my friend’s new rescue dog, Don?
D: Aww, he’s cute. Does he only have three legs?
Y: Yeah, but he’s learned to get along fine without it.
D: Are there any animals that naturally only have three legs? It seems like a lot of animals use only three limbs sometimes. Meerkats rest on their rear legs while supporting themselves with their tails, and woodpeckers use their strong tailfeathers to brace themselves against a tree. And insects have a way of walking where they only use three of their legs that’s called alternating tripod gait—two legs on one side and one on the opposite side stay on the ground, while the other three move.
Y: And don’t forget about parrots, which use their beak as a third, additional grip, and kangaroos, who have a way of walking using their tails and front limbs to move their back legs forward.
D: Using three limbs seems to work fine for a lot of animals. So why haven’t any of them evolved that way?
Y: Well, almost all animals are bilateral—both sides are the same. Take humans, for example, with our two arms and two legs on either side of our body. Scientists think that the code for bilateral symmetry got embedded in animals’ DNA really early on in our evolutionary history, maybe even before appendages like fins, legs, or flippers evolved. Once a trait like that gets written in the DNA, it’s unlikely to change.
D: We can’t escape the past, can we?
Y: At least not when it come to our bilateral symmetry.